This application is a continuation of PCT International Application No. PCT/JP98/05173 filed Nov. 18, 1998.
The disclosures of the following priority applications are herein incorporated by reference: Japanese Patent Application No. 9-316955 filed Nov. 18, 1997 and, Japanese Patent Application No. 10-006117 filed January 14, 1998.
1. Field of the Invention
The present invention relates to a vibration eliminator and an exposure apparatus, and more specifically, it relates to a vibration eliminator capable of reducing vibrations occurring within a structure supported by a vibration eliminating base, an exposure apparatus that employs the vibration eliminator and a projection exposure method achieved by employing the exposure apparatus.
2. Related Art
As a higher degree of accuracy is achieved in high precision apparatuses such as step-and-repeat type exposure apparatuses, i.e., so-called steppers, it has become necessary to block very slight vibrations imparted from the installation floor to the fundamental base (a vibration eliminating base) at a micro-G level. The various types of vibration eliminating pads including a mechanical damper achieved by placing a compressible coil spring in a damping fluid and a pneumatic damper are employed to support the vibration eliminating base of the vibration eliminator, and such a vibration eliminating pad itself has a certain degree of centering function. In particular, a pneumatic spring vibration eliminator provided with a pneumatic damper, which allows the spring constant to be set at a small value and is capable of blocking vibrations of approximately 10 Hz or more is utilized in a wide range of applications for supporting high precision apparatuses. In addition, active vibration eliminators have been proposed in recent years as a means for overcoming the limitations of passive vibration eliminators in the prior art (refer to Japanese Laid-Open Patent Publication No. H8-166043 submitted by the applicant of the present application, for instance). The vibration eliminator disclosed in the publication achieves vibration control by detecting vibrations of the vibration eliminating base with sensors and driving actuators in conformance with the outputs from the sensors and realizes ideal vibration insulation with no resonance peak in the low frequency control range.
In the vibration eliminator described above, the vibration eliminating base control loop is constituted by combining a velocity loop employed mainly for vibration elimination and a position loop implemented for positioning of the main unit. Six vibration sensors and six position sensors are mounted at the vibration eliminating base, and displacement and vibration along six degree of freedom directions detected by these sensors are converted to motion in six degree of freedom directions in the orthogonal coordinate system (coordinate system in mechanical design) which is set as appropriate during the vibration eliminator design stage and, based upon the results of the conversion, damping feedback control is implemented.
In addition, in order to prevent the vibration eliminating base from becoming excited by the reactive force generated as a stage repeats acceleration and deceleration on the vibration eliminating base, a force along the opposite direction achieving the same level as that of the reactive force (counter force) is input through feed-forward so that vibrations caused by stage acceleration and decelerations are reduced while maintaining good blockage of floor vibrations since it is not necessary to increase the position loop gain within the control loop.
The vibration eliminating base described above is designed on the premise that the apparatus mounted on the vibration eliminating base, which constitutes the source of vibration, vibrates together with the vibration eliminating base. In other words, by eliminating vibrations of the vibration eliminating base, which would try to vibrate together with the apparatus constituting the source of vibration, with the active vibration eliminator mentioned earlier, vibration damping is achieved for both the vibration eliminating base and the apparatus mounted at the vibration eliminating base.
The vibration eliminating base is controlled on the premise that the entire vibration eliminating base, i.e., the vibration eliminating base and the apparatus mounted on the vibration eliminating base, vibrates uniformly in a rigid mode without undergoing elastic deformation.
However, as the need for higher performance to be achieved by the vibration eliminating base increases, vibrations occurring on the vibration eliminating base in a local elastic mode can no longer be disregarded. The following is an explanation on vibrations occurring in the local elastic mode. Vibrations occurring in the local elastic mode refer to relative vibrations occurring between an apparatus or a structure mounted on the vibration eliminating base or the like and the vibration eliminating base itself and also local vibrations occurring as a result of elastic deformation of the vibration eliminating base.
If a vibration in the local elastic mode occurs, the vibration detected by a sensor mounted on the vibration eliminating base does not always match the vibration actually occurring on the vibration-eliminating base and, as a result, the effect of active vibration elimination is reduced.
In order to address the problem described above, it is necessary to reduce local vibrations occurring as a result of an elastic deformation of a member constituting the vibration eliminating base (hereafter referred to simply as the xe2x80x9cmemberxe2x80x9d). Vibrations occurring at the member is reduced by increasing the rigidity of the member or by constituting the member with a material achieving a high Young""s modulus and a large damping coefficient such as a high attenuation cast iron, a typical example of which is FCD, in the prior art.
As further miniaturization of semiconductors is achieved necessitating an increase in the semiconductor exposure area, the overall size of exposure apparatuses has become larger. At the same time, the installation conditions under which an exposure apparatus is installed usually do not allow the exposure apparatus to become heavier. Thus, many exposure apparatuses in recent years adopt an open rib structure. There is a problem with such a rib structure, however, in that while the resonance frequency is high, a sufficient degree of rigidity is not achieved and, as a result, vibrations cannot be suppressed.
When a vibration occurs under such circumstances, this vibration in the local elastic mode becomes superimposed on the vibration detected by a sensor of the active vibration eliminator. This poses a concern that the vibration control system of the active vibration eliminator may be caused to vibrate, resulting in the active vibration eliminator exciting the vibration-eliminating base.
This is a problem that may occur at other various apparatuses in which vibrations must be eliminated, as well as at exposure apparatuses.
An object of the present invention, which has been completed to address the problem discussed above, is to provide a vibration eliminator achieving outstanding vibration-eliminating performance by reducing vibrations occurring in the local elastic mode at an apparatus or a structure mounted at the vibration eliminating base or at the vibration eliminating base itself, an exposure apparatus provided with the vibration eliminator and a projection exposure method achieved by employing the vibration eliminator.
In order to achieve the object described above, the vibration eliminator according to the present invention comprises a supporting member; a supported member that is supported by the supporting member; a drive device provided between the supporting member and the supported member, that generates a driving force along the support direction, in which the supporting member supports the supported member; a first vibration detector provided at the supporting member to detect a vibration of the supporting member along the support direction; a second vibration detector provided at the supported member to detect a vibration of the supported member along the support direction; and a damping control circuit that controls the driving force generated by the drive device based upon the vibrations detected by the first and second vibration detectors. According to the present invention, by controlling the driving force generated by the drive device provided between the supporting member and the supported member based upon the results of the supporting member vibration detection and the results of the supported member vibration detection, vibrations occurring at the supporting member and vibrations occurring at the supported member can both be reduced. In addition, since the relative vibrations occurring between the supporting member and the supported member are eliminated, the supporting member and the supported member can be integrated as if they were one rigid unit.
Furthermore, in the vibration eliminator according to the present invention, the damping control circuit reduces relative vibration occurring between the supporting member and the supported member by employing a velocity feedback loop constituted of an integrating circuit and a velocity control circuit to control the driving force generated by the drive device based upon the vibrations detected by the first and second vibration detectors. According to the present invention, since the relative vibration occurring between the supporting member and the supported member is controlled by the velocity feedback loop, optimal damping characteristics for reducing the vibration are achieved. In other words, damping control is achieved as if the vibrations were damped with a viscous resistance (damper). As a result, damping control is facilitated.
The vibration eliminator according to the present invention described above may further comprise a fixed body secured to the installation floor surface and a damping drive device that is provided between the supporting member and the fixed body or between the supported member and the fixed body. The damping control circuit controls the driving force generated by the damping drive device based upon the results of detection of a vibration occurring at the supporting member or the results of detection of a vibration occurring at the supported member. According to the present invention, by implementing damping drive on either one the supporting member and the supported member that are integrated as if they were a single rigid unit with local vibrations reduced, vibrations at the supporting member and the supported member can be reduced in an integrated manner.
The present invention is also adopted in an exposure apparatus that exposes an object to be exposed mounted on a stage with a predetermined pattern via a projection optical system. The exposure apparatus according to the present invention comprises a supporting member; a supported member that is supported by the supporting member; a drive device provided between the supporting member and the supported member, that generates a driving force along the support direction, in which the supporting member supports the supported member; a first vibration detector provided at the supporting member to detect a vibration of the supporting member along the support direction; a second vibration detector provided at the supported member to detect a vibration of the supported member along the support direction; and a damping control circuit that controls the driving force generated by the drive device based upon the vibrations detected by the first and second vibration detectors. According to the present invention, since local vibrations at the exposure apparatus are reduced with a high degree of reliability, the accuracy of the exposure apparatus is improved.
In addition, the present invention is also adopted in a projection exposure method for exposing an object to be exposed mounted on a stage with a predetermined pattern via a projection optical system. In the projection exposure method according to the present invention, relative vibration occurring between a supporting member and a supported member is reduced by controlling the output of a drive device provided between the supporting member and the supported member based upon the results of detection of a vibration in the supporting member occurring along the direction in which the supporting member supports the supported member and the results of a detection of a vibration in the supported member occurring along the support direction. According to the present invention, local vibrations occurring during projection exposure are reduced with a high degree of reliability to achieve highly accurate exposure.
In the projection exposure method according to the present invention, the output from a damping drive device provided between the supporting member and a fixed body or between the supported member and the fixed body is controlled based upon the results of detection of a vibration occurring in the supporting member or the results of detection of a vibration occurring in the supported member. According to the present invention, by reducing local vibrations to integrate the supporting member and the supported member as if they were a single rigid unit and then reducing vibrations by employing the damping drive device, vibrations of the overall apparatus are reduced. Thus, with the local vibrations and the overall vibrations occurring during projection exposure reduced, highly accurate exposure is achieved.
Alternatively, the vibration eliminator according to the present invention comprises a mechanical stress transducer provided at a surface of, or inside a member to convert strain in the member to electricity or magnetism and an energy converter that converts the electricity or the magnetism resulting from the conversion performed by the mechanical stress transducer to thermal energy. Thus, if vibrations cause the strain in the member, the mechanical stress transducer provided at a surface of, or inside the member converts the strain to electricity or magnetism. Then, the electricity or the magnetism resulting from the conversion performed by the mechanical stress transducer is converted to thermal energy by the energy converter. By converting vibrations occurring at the member to thermal energy and dissipating the thermal energy in this manner, vibrations of the member can be reduced.
In the vibration eliminator according to the present invention, the mechanical stress transducer includes a piezoelectric element or an electrostrictive element and the energy converter includes a resistor that shorts an electrode at the piezoelectric element or the electrostrictive element. In the vibration eliminator according to the present invention, if strain occurs due to vibrations in the member, the piezoelectric element or the electrostrictive element included in the mechanical stress transducer provided at a surface of, or inside the member converts the strain to electricity. Then, the electricity resulting from the conversion performed by the piezoelectric element or the electrostrictive element is further converted to thermal energy by the resistor constituting the energy converter. Furthermore, by employing the piezoelectric element or the electrostrictive element achieving a high degree of rigidity to constitute the mechanical stress transducer, no damage occurs or the rigidity of the entire member is not reduced even when the electromagnet transducer is mounted inside the member with a large mass and a relatively high degree of rigidity. By converting a vibration occurring in the member to thermal energy and consuming the thermal energy in this manner, the vibration can be reduced.
In the vibration eliminator according to the present invention, the mechanical stress transducer may include a magnetostrictive element and the energy converter may be provided with a coil in correspondence to the magnetostrictive element and a resistor that shorts the coil. Thus, if strain occurs in the member due to a vibration, the magnetostrictive element included in the mechanical stress transducer provided at a surface of, or inside the member converts the strain to magnetism. Then, the magnetism resulting from the conversion performed by the magnetostrictive element is further converted to electricity by the coil constituting the energy converter, and the electricity is converted to thermal energy by the resistor constituting the energy converter. By converting a vibration occurring in the member to thermal energy and consuming the thermal energy in this manner, the vibration can be reduced. Since the magnetostrictive element achieves a high degree of rigidity, no damage occurs or the rigidity of the entire member is not reduced even when the mechanical stress transducer is installed inside the member with a large mass and a relatively high degree of rigidity.
In addition, the mechanical stress transducer is provided at a position where the member becomes distorted to the greatest extent in the vibration eliminator according to the present invention. By providing the mechanical stress transducer at the position at which the member is distorted to the greatest extent, the conversion of a vibration occurring in the member to thermal energy and the consumption of the thermal energy can be achieved with the maximum degree of efficiency to reduce the vibration with the highest degree of efficiency.
The present invention is also adopted in an exposure apparatus that exposes an object to be exposed with an image of a pattern on a mask via a projection optical system. The exposure apparatus according to the present invention comprises a mechanical stress transducer provided at a surface of, or inside at least one of, a first supporting member that supports the projection optical system, a second supporting member that supports the mask and a third supporting member that supports a substrate, that converts strain in at least one of the first, second and third supporting members to electricity or magnetism and an energy converter that converts the electricity or the magnetism resulting from the conversion performed by the mechanical stress transducer to thermal energy. Thus, if strain occurs at, at least, one of the first, second and third supporting members due to a vibration, the mechanical stress transducer provided at a surface of or inside the supporting member converts the strain to electricity or magnetism. Then, the electricity or the magnetism resulting from the conversion performed by the mechanical stress transducer is converted to thermal energy by the energy converter. By converting a vibration occurring in the member to thermal energy and consuming the thermal energy in this manner, the vibration is reduced.
Alternatively, the vibration eliminator according to the present invention comprises a vibration eliminating base, a structure mounted on the vibration-eliminating base and a local vibration reduction device provided to reduce a vibration that occurs locally within the structure. Since the local vibration reduction device according to the present invention reduces a vibration occurring locally inside the structure, the vibration eliminating performance is improved.
In the vibration eliminator according to the present invention, the local vibration reduction device includes a mechanical stress transducer provided at a surface of, or inside a member constituting the structure, that converts strain in the member to electricity or magnetism and an energy converter that converts the electricity or the magnetism resulting from the conversion performed by the mechanical stress transducer to thermal energy. According to the present invention, by converting the strain in the member constituting the structure to electricity or magnetism and further converting the electricity or the magnetism to thermal energy, a vibration occurring in the member is reduced. Consequently, vibrations occurring locally in the structure can be reduced effectively.
In the vibration eliminator according to the present invention, the local vibration reduction device includes a strain detector that is provided at a surface of, or inside a member constituting the structure and detects strain occurring at the member to generate strain signal; an electrostrictive actuator provided at a surface of, or inside the member, that causes a displacement to an extent that corresponds to the level of an applied voltage; and a voltage control circuit that controls the voltage applied to the electrostrictive actuator based upon the strain signal output by the strain detector. According to the present invention, by controlling the displacement of the electrostrictive actuator provided at a surface of or inside the member in correspondence to the degree of strain in the member detected by the strain detector, a vibration occurring in the member can be reduced effectively.
In addition, in the vibration eliminator according to the present invention, the local vibration reduction device includes a mechanical stress transducer provided at a surface of, or inside a member constituting the structure, that converts strain in the member to electricity or magnetism, an energy converter that converts the electricity or the magnetism resulting from the conversion performed by the electromagnetic conversion element to thermal energy, a strain detector provided at a surface of, or inside the member, that detects strain occurring at the member to generate a strain signal, an electrostrictive actuator provided at a surface of, or inside the member, that causes a displacement to an extent that corresponds to the level of the applied voltage, and a voltage control circuit that controls the voltage applied to the electrostrictive actuator based upon the strain signal output by the strain detector if the absolute value of the strain signal output by the strain detector is larger than a predetermined value. According to the present invention, since the voltage control circuit controls the voltage applied to the electrostrictive actuator based upon the strain signal output by the strain detector if the absolute value of the strain signal output by the strain detector is larger than a predetermined value, in addition to converting strain in the member to electricity or magnetism by employing the mechanical stress transducer and converting the electricity or the magnetism to thermal energy by employing the energy converter, vibrations can be reduced even more effectively.
The present invention is also adopted in an exposure apparatus that exposes an object to be exposed mounted on a stage with a predetermined pattern via a projection optical system. The exposure apparatus according to the present invention comprises a vibration eliminating base, a structure mounted on the vibration-eliminating base and a local vibration reduction device provided to reduce vibrations locally occurring inside the structure. According to the present invention, in which vibrations occurring locally inside the structure constituting the exposure apparatus are reduced by the local vibration reduction device, the accuracy of the exposure apparatus is improved.
The present invention, through which vibrations occurring at a structure on a vibration eliminating base can be reduced, may be adopted in a vibration eliminating base and an exposure apparatus to demonstrate superior damping capability. In addition, the present invention may be utilized in an exposure apparatus that performs exposure by using a charged particle beam, an optical bench such as an interferometer that measures profile irregularities of a test piece with a high degree of accuracy, a vibration-free, ultra precise machine tool and the like.